This invention relates to a mono-axially oriented polyethylene terephthalate (PET) film with extraordinary mechanical strength in the machine direction (MD). A novel process is described that produces film from which tape for various applications can be manufactured. The tape is thinner but stronger than conventional tape.
For some polyester films, high strength is, in general, required in the longitudinal direction, namely the winding direction of the film products, due to application demands. However, resistance against impact and other tearing stresses are also required. Accordingly, in many applications, polyester films or tapes having extremely high tensile strengths in the longitudinal direction with a high tear strength in the transverse direction, sufficient resistance to the stresses of folding, and tearing and impact in all directions, i.e., no tendency towards fibrillation are in demand in the market. Moreover, good dimensional stability or low thermal shrinkage, in addition to the above-mentioned characteristics, is also required.
Films which fulfill the above-mentioned requirements are usually biaxially stretched films of polyethylene terephthalate. In order to stretch film biaxially, however, a transverse (TD) stretching apparatus called a xe2x80x9ctenterxe2x80x9d is required. This apparatus has the drawback not only of high cost, but also requires lower production speed. Moreover, other problems, such as breaking at the time of stretching, or the like are apt to occur. In U.S. Pat. No. 2,556,295 and U.S. Pat. No. 3,627,579, processes for producing films having the above-mentioned properties by uniaxially stretching polyethylene terephthalate are described. According to these processes, tapes or films having less tendency to fibrillation have been obtained by performing the uniaxial stretching in two steps in the first patent is required, while, in the second patent, the use of a polymer with a high degree of polymerization is required. However, these processes have drawbacks also. The first puts more weight on fibers as seen from the examples thereof, but the strength in the transvere direction, required for films, is insufficient and heat shrinkage is also poor. The second process cannot meet both the properties of high strength and antifibrillation completely because these two properties have been shown to be mutually opposite in direction. Further, the strength is insufficient in the direction perpendicular to the stretching direction.
In contrast, British Patent No. 1,136,133 and Japanese Patent Publication No. 29398/1971, describe attempts to improve the drawbacks of conventional uniaxially stretched polyethylene terephthalate films by using polyethylene-1,2-diphenoxyethane-4,4xe2x80x2-dicarboxylate instead of polyethylene terephthalate as a raw material polymer, and uniaxially stretching the layer of the polymer. Of these two patents, the former prevents fibrillation by carrying out a heat treatment to increase the degree of crystallization before MD stretching and the latter prevents fibrillation by dividing MD stretching into two steps and inserting a heat treatment step between these two steps.
However, the processes of these two patents have drawbacks in that, if the degree of preliminary crystallization and the temperature of the intermediate heat treatment are lowered, sufficient reduction of the fibrillation effect cannot be attained. If these parameters are elevated, uniform stretchability becomes worse. This results in non-uniform stretching and strength in the transverse direction (T.D.) is lowered, which occurs because of the preliminary crystallization treatment.
In U.S. Pat. No. 4,286,011 to Wong, there is described the use of a polyester film that includes a mixture of PET and sufficient polytetramethylene terephthalate to increase film tear strength. The film may be unoriented, uniaxially oriented and biaxially oriented in the TD direction only. A tenter frame is used.
In U.S. Pat. No. 3,734,994 to D. L. Blecha, there is described a two stage draw process for producing polyethylene terephthalate film, but there is no heat set step or edge restraint.
The present invention provides a process for producing mono-axially oriented polyethylene terephthalate film having increased mechanical strength in the machine direction which comprises the steps of
drying substantially homopolymer or copolymer polyethylene terephthalate resin having an intrinsic viscosity in the range of about 0.65 to about 0.85 dg/cc and then blending the dried resin with from about 0.5 to about 4.0%, preferably from about 1 to about 3.0% by weight of anti-block additive;
extruding the resin blend to re-melt the resin and make it homogeneous;
casting a film web with the resin blend;
orienting the film immediately after casting it to produce a film of uniform thickness, by drawing the film at a ratio of from about 3.5 to about 4.5 times, preferably about 4.0 times, while constraining the web along its edges to prevent shrinkage in the TD direction;
allowing the film web to relax to a lower tension, cooling the film web, further relaxing the film web and then quenching it to room temperature;
subjecting the film web to a second orientation step under similar conditions to the first step except that the film is drawn at a draw ratio of about 1.025 to about 1.150, and at a draw temperature that is slightly higher than used in the first draw, e.g. about 90 to about 120xc2x0 C., preferably about 100xc2x0 C., preferably at a draw ratio of about 1.05 times while constraining the web along its edges to prevent shrinkage in the TD direction;
allowing the film to relax and annealing the film to a temperature in the range of from about 100 to about 180xc2x0 C., preferably about 150xc2x0 C. and then relaxing it further and cooling it to a temperature near room temperature; and
trimming and winding the film web into rolls.
The process of the present invention produces a product with superior properties. In particular the film does not fibrillate which makes it very useful in the production of tape for a variety of commercial purposes, such as tear tape for packaging. Examples of tear tape applications can be found in U.S. Pat. Nos. 5,806,281 of Krul et al.; 5,730,354 of O""Connor; 5,464,151 of Parker et al.; 4,844,962 of May et al.; and 5,203,935 of May et al. The disclosures of these patents are incorporated herein by reference.
The film is first cast at an ambient temperature that is as low as is feasible and which is dependant on the environmental conditions in which the process is operated. It has been found that the process requires orienting the film immediately after casting, to as high a xe2x80x9cDraw Ratioxe2x80x9d as is practical, usually in the range of about 3.5 to about 4.5 times and preferably 4.0 times, and at a temperature as low as possible, but usually in the range of from about 70 to about 85xc2x0 C., preferably about 70xc2x0 C. without undue force being applied to draw the film web. The film web is held at almost constant width by constraining devices known in the art, examples of which are described in U.S. Pat. No. 4,477,407 issued Oct. 16, 1984 to Hetherington et. al., the disclosures of which are incorporated herein by reference. This patent describes an apparatus and process using a narrow gap draw for orienting a polymeric film. Another example of constraining means known in the art is found in Levy U.S. Pat. No. 4,428,724, the disclosures of which are incorporated herein by reference, which patent describes a microgrooved prcessing roll that reduces the air layer between the polymeric film being processed and the roll. This arrangement aids heat transfer, provides transverse shrinkage restraint and increases processing speed.
After this MD orientation step, the film is trimmed and wound for further processing. At this point in the process, the film has a medium strength in proportion to the draw ratio used. The film is now subjected to a second smaller draw ratio MD orientation process, again holding the film web width constant. This second orientation is conducted at a temperature within a range that will allow the draw force to be kept reasonably low. The film is then relaxed, heat set, relaxed further and cooled. The film is then trimmed and wound in a conventional manner.
The substantially homopolymer or copolymer polyethylene terephthalate resin may be selected from commercially available resins. The substantially or essentially pure homopolymer may contain not more than about 3% by weight of conventionally known impurities, additives or copolymerizing agents. The presence of such material is evident from a drop in the crystalline melting point of the resin which when such impurities are present may range from about 240xc2x0 C. to about 245xc2x0 C., and perhaps up to about 249xc2x0 C. The crystalline melting point for pure homopolymer may range from about 249xc2x0 C. to about 255xc2x0 C. When a copolymer resin is used, its selection is based on whether it can be processed to produce a film having the desired properties. Commercial examples of such materials include KODAK(copyright) 9921 and KODAK(copyright) 9922W.
Surprisingly, the strength of the resulting film is suddenly and dramatically higher after the second orientation than after the first orientation. The film strength is about 20,000 psi stronger in the MD than prior to the second orientation step (in the order of 40% or 1.4 times as high). The film properties obtained are unexpected because the MD draw ratio of the second step is very small and ordinarily one does not expect to see such a large increase in tensile strength. The other unusual result is that the MD tear strength remains high enough for the film to be generally useful as film and does not shrink or fibrillate in post processing applications or use.